Agreed that they should show an interest before you embark on this. Finding joy in general education is more important than job skills for that age.
That said, Ruby and JS are not great languages for teaching fundamentals. They (especially JS) have a lot of weird design decisions that don't translate to other languages and don't map neatly onto problem spaces.
If you insist on JS, start with TypeScript at least. But maybe D, Kotlin, Go, or Dart would be better to teach general concepts.
Python would be a good add-on because they could actually use it to write quick utility scripts and, eventually, data analysis.
Instead of linear programming exercises, it may be easier to find problems they find interesting and help them Google their way through solving them. Maybe they have some tedious homework they could learn to automate.
The problem you'll find is that kids don't do much work, so programming (i.e. making a computer do work for you) isn't necessarily that exciting yet.